1. Field of the Invention
The present invention relates to an ignition coil, and in particular to an ignition coil of a typical internal combustion engine and an ignition coil directly mounted on a plug hole of an engine.
2. Description of the Related Art
In internal combustion engines such as vehicle engines, an ignition coil and an ignition plug are used as detonators that burn gasoline as fuel.
Examples of publications of prior arts relating to an ignition coil for an internal combustion engine include Japanese Laid-Open Patent Publication (Kokai) No. H09-186031 and Japanese Laid-Open Patent Publication (Kokai) No. H11-144986.
FIG. 12 is a longitudinal sectional view showing an ignition coil disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H09-186031. The ignition coil in FIG. 12 is a so-called plug top coil (hereinafter referred to as a “PTC”) and is comprised mainly of a cylindrical coil case 91 having one end thereof opened, a plug cover 92 that is engaged with-an opening in a lower portion of the coil case 91, a low-voltage terminal socket 93 that is mounted on the outside of an upper portion of the coil case 91, a mounting flange 103 that is provided such as to face the low-voltage terminal socket 93, and a coil assembly that is housed in the coil case 91.
The coil assembly is comprised of a primary coil bobbin 95 around which a primary coil 94 is wound, a secondary coil bobbin 97 that is disposed outside and concentrically with the primary coil bobbin 95 and around which a secondary coil 96 is wound, and a bar-shaped core 98 that is inserted into a hollow-body shaft of the secondary coil bobbin 97. A high-voltage terminal mounting portion 99 is formed at a lower end of the secondary coil bobbin 97 of the coil assembly, and an ignition plug 102 is installed so that a distal end thereof is connected to a high-voltage terminal 100 mounted on the mounting portion 99 via a contact 101.
The above constructional members are positioned singly or as an assembly of a plurality of members and then formed as an integral unit by filling thermosetting insulating resin such as epoxy rein into gaps between the constructional members.
FIG. 13 is a horizontal sectional view showing the mounting flange 103 appearing in FIG. 12 and its vicinity.
As shown in FIG. 13, the mounting flange 103 is, for example, integrally molded on an outer peripheral wall of the coil case 91 and has upper and lower two ribs (not shown) in the longitudinal direction of the coil case 91. The upper and lower two ribs ensure the strength of the mounting flange 103 and prevent the mounting flange 103 from becoming deformed during mounting. A top portion of the mounting flange 103 and its vicinity are constructed as a bolt seating 105, in which a bolt hole 106 is formed parallel to the coil case 91.
An igniter 107 is disposed inside the coil case 91 and supported by guide ribs 108 provided on an inner peripheral surface of the coil case 91 which faces the mounting flange 103. The guide ribs 108 guide not only a control unit having the igniter 107 but also an outer peripheral surface of the coil assembly having the primary coil 95 and the secondary coil 96 and positions them in the coil case 91.
In general, a core as a constructional member of an ignition coil is disposed such as to be inserted into a central space of a coil bobbin around which a primary coil or a secondary coil is wound, and in most cases, the core is insertion-molded using mold resin so as to improve corrosion resistance.
FIG. 14 is a longitudinal sectional view showing the relationship between a secondary spool and a cushioning member that is disposed around a central core in an ignition coil disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H11-144986.
As shown in FIG. 14, projections as positioning means 63 for positioning the cushioning member 62 concentrically with and apart from the secondary spool 61 that supports a pair of coils are provided on an inner surface of the secondary spool 61 on the high-voltage side. Also, projections as low-voltage side positioning means 64 for positioning the secondary spool 61 concentrically with and apart from the cushioning member 62 are provided on an outer surface of the cushioning member 62 on the low-voltage side (an outer surface of an upper portion of the cushioning member 62 as viewed in the drawing).
However, the ignition coil disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H09-186031 has the problem that, because the guide ribs 108 for supporting the constructional members are provided on the inner wall surface of the coil case 91 which faces the mounting flange 103, when the ignition coil is fixed, that is, when the ignition coil is fixed at a predetermined location of an engine block by inserting a flange bolt into a bolt hole 106 of the bolt seating 105, the mounting flange 103 and its vicinity become slightly deformed due to fastening stress of the flange bolt, and the deformation reaches the guide ribs 108, and as a result, stress is produced in the hardened insulating resin (casting material) as well, causing the insulating resin to be cracked.
The above problem tends to arise in an ignition coil of the type that a mounting flange itself cannot be made large in size and an ignition coil of the type that, for example, a triangular rib for reinforcement cannot be provided on a mounting flange due to narrowing of a space around an ignition coil mounting portion. In particular, the above problem tends to arise in an ignition coil of the type such as a PTC that is directly mounted on a plug hole of an engine due to many restrictions around the ignition coil.
Moreover, according to Japanese Laid-Open Patent Publication (Kokai) No. H11-144986, the gap between the cushioning member 62 and the secondary spool 61 cannot always be maintained uniform, and there may be a case where stress acting on insulating resin injected between the cushioning member 62 or the central core inserted into the cushioning member 62 and the secondary spool 61 supporting the pair of coils cannot be uniform, and hence the insulating resin is cracked due to distortion.